1. Field of the Invention
The present invention relates to a high-frequency amplifying semiconductor device, in which a MOS (metal oxide semiconductor) field effect transistor and a bipolar transistor are formed within a semiconductor wafer, adapted for use with receivers such as TV, VTR and FM tuners.
2. Description of the Related Art
In a semiconductor device used in a high-frequency amplifier circuit of a tuner, a MOS field effect transistor FET and bipolar transistors are formed in the same substrate such that MOS field effect transistor FET and Darlington-connected bipolar transistors Tr1 and Tr2 are cascade-connected via a diode Di as shown in FIG. 6. Reference numeral 51 denotes an input terminal, 52 a ground terminal, 53 a control terminal, 54 an output terminal, reference character G denotes a gate, D a drain, S a source, E1 and E2 emitters, C1 and C2 collectors, and B1 and B2 bases. As can be seen from a sectional view of the semiconductor device of FIG. 1, use is made of a wafer formed of a P- type Si substrate 1, a P type epitaxial layer 2 and a N.sup.+ type collector buried layer 11. In FIG. 1, reference numeral 3 denotes the base region (N type) of an input protection diode, 4, 5 P.sup.+ type emitter regions of the input protection diode, 6 a P.sup.+ type region of a diode capacitor, 7 an N.sup.+ type region of the diode capacitor, 8 an N.sup.+ type source region of a MOS field effect transistor FET, 9 an N.sup.+ type drain region of the MOS field effect transistor FET, 10 a gate oxide film of the MOS field effect transistor FET, 12 a N type collector region of bipolar transistors TR1, Tr2, 14, 17 P type active base regions of bipolar transistors Tr1, Tr2, 15, 18 P.sup.+ type inactive base regions of bipolar transistors Tr1, Tr2, 16, 19 N.sup.+ type emitter regions of bipolar transistors Tr1, Tr2, 20 a protection oxide film, 21, 22 emitter electrodes of the input protection diode, 23 a polycrystalline semiconductor with no impurities, 24 a polycrystalline semiconductor heavily doped with N type impurities, 25 an upper electrode of the diode capacitor, 26 a wiring electrode between the source of the MOS field effect transistor FET and the diode capacitor, 27 a gate electrode of the MOS field effect transistor FET, 28 a wiring electrode between the drain of the MOS field effect transistor FET and the emitter of the bipolar transistor Tr1, 29 a wiring electrode between the emitter of bipolar transistor Tr2 and the base of bipolar transistor Tr1, 30 a base electrode of bipolar transistor Tr2 and 31 a collector electrode of bipolar transistor Tr2. Those various elements construct the input protection diode, the diode capacitor, the MOS field effect transistor FET and bipolar transistors Tr1 and Tr2.
To obtain a high withstand voltage at the collectors of bipolar transistors Tr1 and Tr2 in the semiconductor device of FIG. 1, N.sup.+ type collector buried layer 11 is formed in P.sup.- type Si substrate 1 and P type epitaxial layer 2 is formed on the buried layer. With such a structure, the source resistance of the MOS field effect transistor FET formed on the same substrate as bipolar transistors Tr1 and Tr2 increases at high frequencies. This is due to the following reason. The source current of the MOS field effect transistor FET flows through a path extending from the source of the FET and electrode 26 of the diode capacitor to a lead frame via a bonding wire and a vertical path extending from the source of the FET and electrode 26 of the diode capacitor to P.sup.- type Si substrate 1 via P.sup.+ type region 6 of the diode capacitor, P type epitaxial layer 2 and P.sup.- type Si substrate 1. An increase in the source resistance results from the existence of P.sup.- type Si substrate 1 in the vertical current path. An increase in the source resistance causes a decrease in high-frequency gain and an increase in noise figure (NF). At high frequencies, current will flow through the vertical current path because the bonding wire exhibits high inductance to impede the current flow.